The invention relates to a new process for the preparation of anthraquinones and substituted derivatives thereof having the general formula: ##STR1## in which R.sub.1, R.sub.2, R.sub.3 and R.sub.4 each represent a hydrogen or halogen atom or a linear or branched alkyl group containing 1 to 5 carbon atoms. These compounds are used for dyestuffs, paper pulp industries and for the manufacture of hydrogen peroxide.
It is known that anthraquinone can be produced industrially by oxidation of anthracene. This process, however, is dependent upon problems of supply of anthracene from coal tar. It has been proposed to prepare anthraquinone from 1,4-naphthoquinone and butadiene (British Pat. No. 895,620) but the processes for obtaining 1,4-naphthoquinone are complicated and costly (British Patent No. 1,499,068).
Another much used industrial method enables anthraquinone and the substituted anthraquinones to be prepared from phthalic anhydride and benzene in the presence of aluminum chloride (U.S. Pat. No. 1,656,575), but this method has the disadvantage of the very considerable cost of the aluminum chloride which is consumed at the rate of 2 moles of aluminum chloride per mole of phthalic anhydride. In order to alleviate this disadvantage it has been proposed to react the gaseous mixture at high temperature over a solid catalyst based on a silico-aluminate (Japan Kokai Sho 49/30350 and Sho 59/95952) or on titanium oxide (Japan Kokai Sho 54/70252) but these processes impose the operation in the gaseous phase at high temperature and necessitate a complex installation with a costly investment.
More recently, U.S. Pat. No. 4,379,092 discloses forming anthraquinones by a two-step process. First, an equimolecular mixture of HF and BF.sub.3 is used to catalyze the condensation of phthalic anhydride and benzene to o-benzoylbenzoic acid. The patent states that the reaction proceeds "rather surprisingly, without causing cyclization of the benzoylbenzoic acid which is formed, therefore without liberating a molecule of water which would hydrolyze or hydrate BF.sub.3 and thus make the recovery of the catalyst difficult." Moreover, the patent states the fact that the equimolecular mixture of HF and BF.sub.3 does not cause cyclization is quite unexpected since the cyclization of benzoylbenzoic acid to anthraquinone takes place in anhydrous HF (U.S. Pat. No. 2,174,118). Secondly, the pure o-benzoylbenzoic acid is obtained by extraction of the crude reaction mixture with boiling water, followed by recrystallization by cooling and conversion of the o-benzoylbenzoic acid into the anthraquinone by heating in concentrated sulfuric acid or by any other means of cyclization known in the art. The cyclization may take place in the course of the production of intermediate anthraquinone compounds for dyes wherein the sulfonation and nitration reactions take place in a cyclizing medium, for example H.sub.2 SO.sub.4 or HF.
While the process described in U.S. Pat. No. 4,379,092 for producing anthraquinone compounds is useful, the need to separate the benzoylbenzoic acid from the reaction medium for subsequent cyclization is a disadvantage and adds additional costs to the overall process. Accordingly, it is an object of the present invention to produce anthraquinones by the condensation of phthalic anhydride and benzene compounds and avoid the separation and recovery of intermediate products.